1. Field of the Invention
The present invention relates to an improvement of a female thread processing method of screwing a cut tap into a prepared hole so as to cut a female thread.
2. Discussion of Related Art
A female thread processing method of screwing a cut tap into a prepared hole so as to cut a female thread is widely applied to various fields. However, in the case of tapping a material having a high ductility, for example, SS400 (an ordinary structural rolled steel), an aluminum and the like by using the cut tap, the chips are hard to be broken so as to tend to be connected long. Accordingly, there is a case that the chips entwine the tap so as to make the processing impossible as shown in FIG. 10. Therefore, it is necessary for an operator to always monitor, and there is a problem that it is hard to achieve a complete automation. Further, there is a risk that the tap is broken due to pinching of the chips, and there are various problems such that it is hard to execute a process such as a cleaning work of the chips, and the like. On the contrary, in the case that a plurality of notches corresponding to a land of the cut tap are provided in an outer peripheral portion of the prepared hole, as described in patent JP-A S52-26698, the chips are broken in pieces by the notches. Accordingly, the problems mentioned above can be solved.
However, in the processing method described in the patent document 1 mentioned above, since the female thread itself is largely broken by the comparatively large notches corresponding to the land of the cut tap, there is a risk that a strength of the female thread is lowered, and a predetermined screwing strength can not be obtained. In particular, since it is impossible to strongly fasten the material in the case of the material having the high ductility, it is substantially hard to apply the technique described in the JP-A S52-26698.